Project Summary Our intestine is host to a complex microbial community, the gut microbiota, which is dominated by obligate anaerobic bacteria belonging to the classes Clostridia and Bacteroidia. It is well established that a disruption of the gut microbiota by antibiotic treatment leads to an expansion of facultative anaerobic Enterobacteriaceae, including pathogens, such as Salmonella enterica serovar Typhimurium (S. Typhimurium), however, the underlying mechanisms are just beginning to be worked out. Importantly, it is not known whether S. Typhimurium expands and/or triggers changes in the microbiota composition in the absence of antibiotic treatment. Our long-range goal is to elucidate molecular mechanisms that control the balance between the pathogen, the host and its gut microbiota. The objectives of this application are to study the mechanisms that enable the pathogen to gain an edge over competing bacteria during intestinal inflammation. Our central hypothesis is that in the absence of antibiotic treatment, S. Typhimurium virulence factors trigger host responses that lead to a depletion of butyrate-producing Clostridia from the gut-associated microbial community, which in turn increases oxygenation of colonocytes, thereby driving a cytochrome bd-II oxidase- dependent pathogen expansion in the gut lumen. We will test different aspects of our hypothesis and accomplish the objectives of this application using the logical and innovative approach outlined in the following specific aims: Specific Aim 1: Determine how S. Typhimurium depletes Clostridia during colitis. Specific Aim 2: Determine how Clostridia depletion drives S. Typhimurium growth during colitis. Specific Aim 3: Determine whether respiration drives S. Typhimurium transmission. It is our expectation that successful completion of the proposed experiments will usher in important conceptual advances in understanding the mechanisms underlying pathogen expansion during S. Typhimurium-induced gastroenteritis. This paradigm-shifting work will be significant because it will have wide appeal among researchers interested in microbial pathogenesis and the contribution of host-associated microbial communities to health and disease.